1. Field of the Invention
The invention relates to a novel oil-based graphite lubricant for the plunger of a die-casting machine. The invention further relates to a production method of such a lubricant.
2. Description of Related Art
A lubricant is used to suitably lubricate the sliding surfaces between the shot sleeve and the plunger tip on the plunger in a die-casting machine. Plunger lubricants exist in a variety of types, including oil-based lubricants, water-soluble lubricants, and water-in-oil (w/o) emulsion-type lubricants. Of these, oil-based graphite lubricants obtained by compounding graphite in a base oil are widely used, both because of their high lubricating ability and because they can easily form a uniform film on metal surfaces. However, conventional oil-based graphite lubricants have certain drawbacks. For example, the graphite contained within the lubricant tends to aggregate and settle out during storage, resulting in variable lubricity, in addition to which the oil that rises to the top thermally decomposes and gasifies. Re-dispersing the graphite that has aggregated and settled out requires extensive stirring in a ball mill or the like.
In the field of casting, graphite-containing release agents are disclosed in, for example, Japanese Patent Application Publication No. 5-7978 (JP-5-7978 A), Japanese Patent Application Publication No. 2000-33457 (JP-2000-33457 A) and Japanese Patent Application Publication No. 11-244992 (JP-11-244992 A). In JP-5-7978 A, an aromatic surfactant is used to ensure the dispersibility of the graphite powder. In JP-2000-33457 A, a solid lubricant (graphite, etc.) in powdered form is given a small particle size and thereby rendered into a uniformly dispersed state within a liquid. JP-11-244992 A discloses the mixture of sooty graphite with a sol-like binder obtained by dispersing a feathery heat-resistant inorganic hydrate in water.
However, even when referring to JP-5-7978 A, JP-2000-33457 A and JP-11-244992 A, producing a release agent containing stably dispersed graphite which does not aggregate and settle out is a challenge. Moreover, because these literature references describe release agents, it is not obvious whether the disclosures made in these references are in any way applicable to a plunger lubricant, for which different properties are required.